The ability to detect chemical analytes, especially organic chemical analytes, is important in many applications, including environmental monitoring, and the like. Such detection and/or monitoring of organic molecules may find particular use in, for example, so called End of Service Life Indicators which are desired for personal protective equipment such as respirators.
Many methods for the detection of chemical analytes have been developed, for example optical, gravimetric, microelectromechanical, and so on. In particular, sensors that monitor electrical properties such as capacitance, impedance, resistance, etc., have been developed. Often, such sensors rely on the change that occurs in the electrical properties of a material upon adsorption of an analyte onto, or absorption of an analyte into, the material.
For example, U.S. Patent Application Publication 2006/0249402 to Snow et al. discloses a sensing device having a bottom electrode, a dielectric on the bottom electrode, a grid of nanoelectrodes on the dielectric, and a top electrode in electrical contact with the grid. The nanoelectrodes may be a network of carbon nanotubes. Such an arrangement is described by Snow et al. as being capable of exhibiting a capacitance change in the presence of a chemical analyte.
U.S. Patent Application Publication 2006/0237310 to Patel et al. discloses a device that is described as being able to detect various target analytes by adsorption or absorption of the analyte in a chemical sensing material such that an electrical property (e.g. capacitance, resistance, etc.) is altered in a manner detectable by circuitry associated with the sensing electrode pair coated with the chemical sensing materials.
U.S. Pat. No. 5,512,882 to Stetter and Maclay discloses a sensor whose impedance changes upon exposure to a vapor of a selected chemical substance. The sensor comprises a polymer whose physical structure is altered by the vapor (e.g., through expansion or disintegration). The sensor further comprises electrically conductive elements that are interspersed with the polymer. Changes can be measured by an impedance-measuring circuit.
U.S. Pat. No. 5,482,678 to Sittler discloses a sensor comprising a material which expands in the presence of an organic liquid, gas or vapor. The material is applied to a support surface such that upon expansion, the support deflects and changes the distance between two capacitor plates, thereby changing an electrical capacity between the plates.
U.S. Pat. No. 5,965,451 to Plog and Maunz discloses a gas sensor for selective detection of hydrocarbons, having a capacitive element and a gas-permeable sensitive layer as a dielectric. The sensitive layer is precious-metal-doped zeolite which has a regular crystalline structure made of primary pores whose diameter is on the order of the gas-kinetic diameter of the gas molecules to be detected.